The present invention relates to a liquid crystal display, in which light intensity is controlled by amplitude of voltage applied thereto, and in particular to a liquid crystal light valve suitable for a projection type display and a projection type display using same.
Liquid crystal displays of active matrix system using MOS (Metal Oxide Semiconductor) transistors formed on a monocrystal silicon substrate for switching elements are described in U.S. Pat. No. 3,862,360 and Technical Reports of the Electronic Communication Society, IE 80-81, 1980, in which the switching elements and a liquid crystal layer are superposed on each other and light intensity is controlled by means of the former.
All these displays are of a system in which images obtained by controlling the switching elements are viewed directly, and they are used usually in a room. For this reason, a resistance to light intensity of several tens of thousand 1× necessary for a display panel was sufficient.
When an MOS transistor is irradiated with light, photo-current is produced in PN junction portions forming a source and a drain in the MOS transistor. This current changes voltage applied to the liquid crystal, which remarkably worsens image quality.
In order to reduce this photo-current, according to the Technical Reports of the Electronic Communication Society described previously, various countermeasures were taken that the source region in the MOS transistor was located as far as possible from a region where light was injected, that the surface of the silicon substrate on which MOS transistors were formed was covered by two wiring layers, that a stopper diffusion layer was disposed to recombine generated carriers, etc.
In addition, since display size in the displays described above was as small as about 5 cm because of restriction imposed by silicon wafers, etc., the number of pixels in such a display was about 40,000 due to this display size and resolving power capable of producing recognizable images.
As described above, liquid crystal displays using MOS transistors formed on a monocrystal silicon substrate were restricted to be of direct view type.
On the other hand, in a projection type display, a panel constructed by superposing switching elements and a liquid crystal layer on each other is called a liquid crystal light valve and images controlled by this light valve are projected on a screen in an enlarged scale. For this reason intensity of light projected to the light valve should be increased, corresponding to the enlargement on the screen, and brightness thereof attains several million 1×. Furthermore since the pixels controlled by the light valve are enlarged and the images thus obtained are roughened, more than 30,000 pixels are required for the light valve.
As described above, for a projection type display, such as where a liquid crystal light valve using transistors is formed on a semiconductor substrate such as silicon substrate, it is required to increase resistance to light intensity of the liquid crystal light valve and to write image signals with a high speed in pixels due to increase in the number of pixels.